ARTICLE

Atacamite formation by deep saline waters in copperdeposits from the Atacama Desert, Chile: evidence from fluidinclusions, groundwater geochemistry, TEM, and 36Cl data

Martin Reich & Carlos Palacios & Miguel A. Parada &

Udo Fehn & Eion M. Cameron & Matthew I. Leybourne &

Alejandro Zúñiga

Received: 18 December 2007 /Accepted: 13 March 2008 / Published online: 27 May 2008# Springer-Verlag 2008

Abstract The presence of large amounts of atacamite inoxide zones from ore deposits in the Atacama Desert ofnorthern Chile requires saline solutions for its formationand hyperarid climate conditions for its preservation. Weinvestigated the nature and origin of atacamite-formingsolutions by means of coupling groundwater geochemicalanalyses with fluid inclusion data, high-resolution mineralo-gical observations, and chlorine-36 (36Cl) data in atacamitefrom the Mantos Blancos and Spence Cu deposits. In bothdeposits, the salinities of fluid inclusions in atacamite are

comparable to those measured in saline groundwaterssampled from drill holes. The average salinity of fluidinclusions in atacamite for the Mantos Blancos and Spencedeposits (~7–9 and 2–3 wt.% NaCleq, respectively) arestrongly correlated to the salinities at which gypsumsupersaturates from groundwaters in both deposits (totaldissolved solids ~5–9 and 1–3 wt.% NaCleq, respectively).This correlation is confirmed by transmission electronmicroscopy observations of atacamite-bearing samples,revealing an intimate association between atacamite andgypsum that can be traced down to the nanometer scale.36Cl data in atacamite provide new lines of evidenceconcerning the origin and age of the saline waters thatformed atacamite in various stratabound and porphyry Cudeposits from the Atacama Desert. All atacamite samplesshow very low 36Cl-to-Cl ratios (11!10!15 to 28!10!15 atat!1), comparable to previously reported 36Cl-to-Cl ratios ofdeep formation waters and old groundwaters. In addition,36Cl-to-Cl ratios in atacamite correlate with U and Thconcentration in the host rocks but are independent fromdistance to the ocean. This trend supports an interpretationof the low 36Cl-to-Cl ratios in atacamite as representingsubsurface production of fissiogenic 36Cl in secularequilibrium with the solutions involved in atacamiteformation. Therefore, 36Cl in atacamite strongly suggestthat the chlorine in saline waters related to atacamiteformation is old (>1.5 Ma) but that atacamite formationoccurred more recently (<1.5 Ma) than suggested inprevious interpretations. Our data provide new constraintson the origin of atacamite in Cu deposits from the AtacamaDesert and support the recent notion that the formation ofatacamite in hyperarid climates such as the Atacama Desertis an ongoing process that has occurred intermittently sincethe onset of hyperaridity.

Miner Deposita (2008) 43:663–675DOI 10.1007/s00126-008-0184-4

Editorial handling: B. Lehmann

M. Reich (*) : C. Palacios :M. A. ParadaDepartamento de Geología,Facultad de Ciencias Físicas y Matemáticas,Universidad de Chile,Santiago, Chilee-mail: mreich@cec.uchile.cl

U. FehnDepartment of Earth and Environmental Sciences,University of Rochester,Rochester, NY 14267, USA

E. M. CameronEion Cameron Geochemical Inc.,865 Spruce Ridge Road,Carp, Ontario K0A 1L0, Canada

M. I. LeybourneOcean Exploration, GNS Science,Lower Hutt, New Zealand

A. ZúñigaDepartamento de Ingeniería Mecánica,Facultad de Ciencias Físicas y Matemáticas,Universidad de Chile,Santiago, Chile

Introduction

Atacamite and its polymorph paratacamite (Cu2Cl(OH)3)are major constituents of supergene oxide zones in manyCu deposits from the Atacama Desert of northern Chile(Sillitoe and McKee 1996; Chávez 2000; Mote et al. 2001;Bouzari and Clark 2002; Hartley and Chong 2002; Dunai etal. 2005; Hartley and Rice 2005; Sillitoe 2005; Rech et al.2006). Atacamite formation has been traditionally inter-preted as a primary product of supergene oxidation (e.g.,Chávez 2000), formed after leaching of Cu sulfides byoxygenated meteoric waters that percolated through thedeposits. Supergene oxidation of copper porphyry depositsoccurred over a long period from 44 to 9 Ma, after whichthe climate became hyperarid (Arancibia et al. 2006).Atacamite requires saline water for its formation (Hannington1993), rather than fresh meteoric water. Recognizing this,Arcuri and Brimhall (2003) proposed that atacamite in theRadomiro Tomic deposit, where this is the principal oxidemineral, was formed during supergene oxidation bypercolating saline meteoric water. However, atacamitedissolves rapidly or undergoes phase change when exposedto fresh, meteoric water. During the long period prior to theonset of hyperaridity, when oxide zones were exposed topercolating rainwater, any atacamite would have beenremoved. Moreover, some oxide zones containing ataca-mite were later covered by piedmont gravels. Stream waterscarrying these gravels would have permeated through theoxide zones, removing atacamite. Based on these argu-ments, Cameron et al. (2007) suggested that atacamite-bearing oxide assemblages are more likely to have formedafter the onset of hyperaridity by the replacement ofpreexisting oxide assemblages, either by earthquake-induced pulses of deep saline waters through the depositsor by meteoric waters which became saline by evaporationat the surface.

When hyperaridity commenced is much debated. Previ-ously, it was thought to have followed the main period ofsupergene enrichment ending at 14 Ma (Alpers andBrimhall 1988; Sillitoe and McKee 1996). However, recentdating of post-14-Ma supergene events and other evidence(Hartley and Rice 2005; Arancibia et al. 2006) suggest thathyperaridity started as late as 4 Ma. Leybourne andCameron (2008) provide evidence that atacamite and othersecondary Cu minerals may currently be forming byoxidation of hypogene and/or supergene sulfides, suggest-ing that supergene Cu mineralization is possible even underthe hyperarid conditions that dominate the present-dayclimate of the Atacama Desert.

The aim of this study is to provide further insight intothe origin and source of waters that form(ed) atacamite byintegrating mineralogical and isotopic data of atacamitewith groundwater chemistry data from the Mantos Blancos

and Spence Cu deposits in the Atacama Desert of northernChile. We report salinity data of fluid inclusions trapped inatacamite, and we compare these results with groundwatersalinities. In addition, we complement these data with high-resolution transmission electron microscopy (TEM) obser-vations of atacamite samples and 36Cl isotopic data ofatacamite from various deposits in the Atacama Desert.

Geologic setting and atacamite occurrence

About 40% of the known Cu resources in the world occurin Chile, and the hyperarid Atacama Desert of northernChile contains some of the world’s largest Cu deposits(Maksaev et al. 2007). Supergene oxidation and leachingprocesses have produced important enrichment of porphyryand stratabound (“manto-type”) Cu deposits, and theeconomic viability of these deposits is typically dependenton the size and quality of the supergene enrichment blanketthat overlies the hypogene Cu–sulfide zones (Sillitoe andMcKee 1996; Maksaev et al. 2007). Notable occurrences ofatacamite (with variable proportions of other oxide miner-als) are found in several Cu deposits in northern Chile. Thelocation of atacamite-bearing deposits, including MantosBlancos and Spence, is shown on Fig. 1, and the geology ofthese deposits has been recently reviewed by Ramirez et al.(2006) and Cameron et al. (2007). Both Spence and MantosBlancos are located along the prominent Antofagasta–Calama Lineament (ACL, Fig. 1), a major structural feature

Fig. 1 Map showing the location of various Cu deposits in theAtacama Desert in northern Chile, after Cameron et al. (2007). Blacksquares and circles represent atacamite-containing porphyry Cudeposits and other Cu deposits, respectively (white squares are otherporphyry deposits). Physiographic zones (Coastal Ranges, CentralDepression, Precordillera, and High Andes) are depicted in shades ofgray, and DFZ and ACL are the Domeyko Fault Zone and theAntofagasta-Calama Lineament (Palacios et al. 2007)

664 Miner Deposita (2008) 43:663–675

that is thought to have controlled deformation and tectonicrotation in the region since Early Cretaceous (Ramirez et al.2006). The Spence porphyry Cu deposit contains 231 Mt ofsulfide ore at 1.13% Cu and 79 Mt of oxide ore (atacamiteand other oxides) at 1.18% Cu (Cameron et al. 2007).Porphyry intrusion and hypogene mineralization (in theporphyries and adjacent andesites) at Spence took placeduring the Paleocene, at around 57 Ma (Rowland and Clark2001). The Mantos Blancos porphyry-like deposit also lieson the ACL, and the oxidized ore represents ~30% of thetotal Cu reserve (200 Mt extracted since 1960; Maksaev etal. 2007). The main ore formation has been related to anUpper Jurassic hydrothermal event at ~140 Ma and consistsof hydrothermal breccias, disseminations, and stockworksassociated with sodic alteration (Ramirez et al. 2006).Mantos Blancos and Spence deposits are characterized bywell-developed atacamite-bearing oxide zones and arecovered by ~10–100- and 50–100-m-thick gravels andgypsum-rich saline soil, respectively. The current watertables at the Mantos Blancos and Spence deposits arelocated at depths of ~400 and 50 to 90 m below the surface,respectively.

Atacamite is the major component of oxide zones ofmany Cu deposits in the Atacama Desert (Fig. 1). Ittypically occurs as complex, fine-grained cryptocrystallineaggregates forming veinlets, disseminations and/or replac-ing preexisting Cu minerals as corrosion coatings, althoughit occasionally occurs as coarser, millimeter-scale crystals.Atacamite has also been reported as a secondary mineral inthe weathered portions of massive sulfide deposits on themodern seafloor (Hannington 1993). Hannington (1993)described atacamite in TAG mounds as occurring in patchycolloform masses, as crystalline aggregates that fill openspaces, and as disseminated crystals intergrown with fine-grained Fe-oxy-hydroxides. The crystalline habits of ataca-mite include prismatic crystals and equant, rhombic prisms.Individual euhedra range in size from a few tens ofmicrometers to about 2 mm but are typically less than500 μm in size (Hannington 1993). Atacamite commonlyassociates with gypsum in the supergene zones of Cudeposits (e.g., at Radomiro Tomic; Cuadra and Rojas et al.2001) as well as in submarine hydrothermal ores (Glynn etal. 2006). Sedimentary textures described by Glynn et al.(2006) in supergene alteration zones of submarine hydro-thermal sulfides indicate that gypsum and atacamiteprecipitation may be contemporaneous.

Materials and methods

Groundwater samples collected from drill holes at theMantos Blancos deposit were analyzed for major and minorcations (Na, K, Ca, Mg, Si, Cu, Fe; inductively coupled

plasma optical emission spectrometry) and anions (Cl, SO4,NO3; ion chromatography) at Anglo American Laboratories.

Thermometric measurements in primary fluid inclusionstrapped in atacamite from the Mantos and Spence depositswere conducted using a Linkam THM-600 stage at theFluid Inclusions Laboratory, Departamento de Geología,Universidad de Chile. The stage was calibrated with liquidCO2 inclusions, distilled mercury, and pure water inclusions(accuracy of the melting point temperatures is estimated tobe ±0.4°C). Only freezing temperature measurements wereperformed. Heating experiments to determine the homoge-nization temperature of the inclusions were avoided,considering the fact that temperatures obtained fromheating experiments may be considerably higher than thetrue values, due to stretching of fluid inclusions trapped insoft minerals such as atacamite (Moh’s hardness ~3–3.5).Stretching due to overheating of fluid inclusions in softminerals (Moh’s hardness <3.5) has been extensivelydocumented by many authors, including Bodnar andBethke (1984), Ulrich and Bodnar (1988), and Vanko andBach (2005).

Atacamite in oxide zones from ore deposits typicallyoccurs as very fine-grained aggregates in veins and/ordisseminated. None of the traditional optical and microana-lytical techniques can image the structure and textures ofatacamite aggregates below the micrometer level, nor itsmineralogical associations at that scale. To investigate themicrometer-to-nanometer texture of fine atacamite aggre-gates from the Mantos Blancos and Spence deposits, TEMobservations were performed at the Laboratorio de Micro-scopía Electrónica de Transmisión, Departamento de Geo-logía, Universidad de Chile, Santiago, Chile. TEM sampleswere cut from polished thin sections and final thinning wasperformed by ion milling with an Ar beam (4.0 keV) in aGatan precise ion-polishing system. Details on TEM samplepreparation are presented elsewhere (Reich et al. 2005).Observation was carried out using a FEI Tecnai F20 FEGTEM operated at 200 kVequipped with an energy-dispersivespectrometry (EDS) detector. Radiation beam damage wasnot observed during the analyses. Details on the TEMtechnique can be found in Williams and Carter (1996).

Because of the extremely low abundance of 36Cl innature, detection of this isotope is only possible byaccelerator mass spectrometry (AMS). Isotope determina-tions of 36Cl in atacamite and water from recent rain eventsin the area were conducted using AMS at PrimeLab, PurdueUniversity, USA, following the established extraction andanalytical methods reported by Fehn et al. (1992) andSharma et al. (2000). The detection limit is around 1!10!15

for the 36Cl-to-Cl ratio with a typical precision of 5–10%(1σ), which decreases, however, considerably for samplesclose to the detection limit or where carrier material hasbeen used.

Miner Deposita (2008) 43:663–675 665

Results

Groundwater salinity

The Mantos Blancos groundwaters span a wide range ofsalinities (Table 1) and can be subdivided into three maingroups based on TDS values: (a) low-salinity waters (TDS<10,000 mg/L), represented by only one sample (14877); (b)saline waters (10,000<TDS<100,000 mg/L, 16 samples);and (c) high-salinity waters (TDS>100,000 mg/L, 15samples). There is about two orders of magnitude differencein the Cl! and Na+ abundance of the three types of waters, aslow-salinity waters show values of 2,050 mg/L Cl! and1,570 mg/L Na+, whereas the saline waters and high-salinitywaters average 18,376 and 76,251 mg/L Cl! and 17,779 and46,223 mg/L Na+, respectively. Mantos Blancos ground-waters are mainly Na+–Cl––SO2!

4 -type waters, and anions

are dominated by Cl!, with all samples showing a strongpositive correlation between Na+ + Cl! and TDS (Fig. 2a),over the entire range in TDS. A similar trend of increasingSO2!

4 with TDS is observed for low-salinity and salinewaters, but this correlation breaks down for high-salinitywaters, where a strong decrease inSO2!

4 with increasing TDSis observed (Fig. 2b). Similar trends have been reported forgroundwaters at the Spence deposit by Leybourne andCameron (2006).

Fluid inclusion salinity

All of the fluid inclusions observed in atacamite from theSpence and Mantos Blancos deposits consist of liquid andvapor (L + V) with no daughter mineral. Most inclusionshave diameters of <30 μm, with <30% vapor by volume.Inclusions are scarce and are typically found scattered

Table 1 Major and minor ions of groundwaters from Mantos Blancos deposit, northern Chile

Sample Cl,mg/L

SO4,mg/L

NO3,mg/L

Na,mg/L

K,mg/L

Ca,mg/L

Mg,mg/L

Si,mg/L

Cu,mg/L

Fe,mg/L

pH TDS,mg/L

TDSa,wt.%NaCleq

14877 2,050 1,250 0.8 1,570 20 290 20 4.0 1.7 0.7 8.0 6,200 0.3614253 11,850 8,220 4.6 11,470 70 730 100 8.6 2.1 0.1 7.9 37,900 2.3314251 11,550 7,680 4.3 11,230 70 700 110 11.3 2.5 0.3 7.9 38,100 2.2814250 14,100 9,070 4.4 13,440 80 780 130 10.4 2.4 0.2 8.0 42,200 2.7514249 13,800 9,090 3.6 13,070 80 750 150 11.0 2.4 0.1 7.9 42,400 2.6914267 12,800 9,800 4.7 12,460 70 690 120 13.9 2.7 0.2 7.5 43,300 2.5314268 17,000 10,590 4.5 15,960 90 740 200 13.6 2.9 1.9 7.7 52,100 3.3014270 18,800 12,130 5.1 17,720 90 740 240 14.0 2.2 0.1 7.8 57,400 3.6514269 23,300 10,410 4.6 18,600 90 860 320 13.9 3.0 0.1 7.6 61,800 4.1914271 22,700 11,600 4.8 19,470 90 800 310 18.5 5.5 0.3 7.5 62,500 4.2214751 20,010 14,860 6.9 20,140 160 600 290 18.0 1.5 0.7 7.2 63,000 4.0214272 19,900 14,870 6.5 20,350 90 670 340 12.8 2.8 0.3 7.5 66,500 4.0314752 20,480 15,130 7.0 20,540 170 620 310 18.0 1.3 0.7 7.3 66,900 4.1014755 21,020 15,310 7.3 21,270 170 620 330 19.0 1.7 0.7 7.2 67,000 4.2314753 20,860 15,190 7.1 21,150 160 620 320 21.0 1.5 0.7 7.3 67,400 4.2014754 20,790 15,330 7.1 21,380 170 630 320 20.0 1.5 0.7 7.2 67,800 4.2214952 25,050 25,600 10.0 26,210 260 640 2,290 6.0 1.9 6.7 7.8 93,200 5.1314894 55,710 4,560 5.9 35,920 620 950 2,070 18.0 0.9 2.8 7.5 110,100 9.1614252 61,500 1,280 9.7 28,530 130 9,220 1,510 20.1 3.7 1.0 6.9 115,800 9.0014933 55,950 9,890 7.0 37,200 370 1,140 1,350 4.0 0.4 2.1 7.2 116,100 9.3213877 55,320 10,000 5.9 38,680 320 1,040 1,030 10.8 0.9 0.8 7.5 116,400 9.4014285 57,400 1,700 13.5 28,250 100 6,300 1,420 23.1 4.7 3.2 7.3 122,300 8.5714284 68,300 1,750 12.7 35,710 120 6,180 1,420 15.4 5.4 1.0 6.9 137,900 10.4014912 69,230 9,680 5.7 48,760 480 1,180 1,980 10.0 0.6 7.5 7.0 138,400 11.8014876 69,540 10,520 4.6 45,720 290 960 1,700 4.0 2.6 5.1 7.5 138,600 11.5314936 68,680 10,340 8.7 48,100 460 1,200 1,530 6.0 0.6 4.6 7.3 143,500 11.6814283 68,400 1,410 12.0 32,860 160 8,000 1,700 19.6 5.0 1.1 6.9 144,000 10.1314935 76,410 10,430 9.9 51,200 520 1,240 1,770 2.0 0.6 3.9 7.3 155,100 12.7614932 80,330 10,080 7.5 50,610 500 1,180 2,130 4.0 0.7 2.9 7.3 156,200 13.0913966 107,500 6,650 5.5 62,950 580 880 2,700 12.5 1.9 2.0 7.3 192,100 17.0513967 124,000 6,250 6.5 74,050 640 890 3,550 2.0 1.3 2.5 7.0 229,900 19.8113965 125,500 6,650 6.5 74,800 640 870 3,350 2.0 1.0 2.5 7.0 230,200 20.03

a TDS*[weight percent NaCleq]=(Na[milligram per liter] + Cl[milligram per liter])10!4

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throughout atacamite crystals, forming clusters (Fig. 3a–d).Due to the scarcity and small size of the inclusions, coupledwith the fact that atacamite occurs as polycrystallineaggregates of fine-grained crystals and does not showgrowth zones under polarized light, the primary orpseudosecondary nature of the inclusions could not beconfirmed (secondary inclusions were not observed). Thesalinity of the fluid inclusions was determined using the

formula reported by Bodnar (1993) for the NaCl–H2Osystem, as no evidence was observed for either liquid CO2

or clathrate formation on freezing point depression mea-surements. Salinities of 48 fluid inclusions trapped withinfive atacamite samples from Mantos Blancos (each one rep-resenting different veinlets) vary between 6.4 and 10.7 wt.%NaCl equivalent (NaCleq, with an average of 8.4 wt.%NaCleq; Fig. 4; Table 2). In contrast, the salinities of 43 fluid

Fig. 2 Plots of total dissolved solids (TDS) versus Na + Cl cations (a) and SO4 anions (b) for groundwaters from Mantos Blancos and Spencedeposits. Data from Spence taken from Leybourne and Cameron (2006)

Fig. 3 Photomicrographs offluid inclusions in atacamitefrom a Mantos Blancos sample,taken in plane polarizedreflected light (uncrossedpolars). Rectangle in a showsmagnified area in b. All of thefluid inclusions observed in ata-camite from Spence and MantosBlancos deposits (diameters<30 μm, a–d) consist of liquidand vapor (L + V) with nodaughter mineral (c and d).Inclusions are scarce and areusually found scatteredthroughout atacamite crystals,forming clusters

Miner Deposita (2008) 43:663–675 667

inclusions, determined in seven samples of atacamite-bearingveinlets at Spence, vary between 1.7 and 4.3 wt.% NaCleq,averaging 2.7 wt.% NaCleq (Fig. 4).

TEM observations

Atacamite occurs as polycrystalline aggregates associatedwith gypsum in veinlets (Fig. 5a). TEM observations in aselected atacamite–gypsum veinlet sample from the Spencedeposit (Fig. 5a) reveal that atacamite is in close associationwith gypsum at all scales of observation (hand sample tonanoscale, Fig. 5a–e) The presence of atacamite andgypsum at the nanoscale was confirmed using EDS spotanalyses. TEM images show dark, opaque, and elongatedcrystals of atacamite (<10 μm in size) associated withlighter gray, translucent gypsum crystals that show itscharacteristic cleavage (Fig. 5c). Figure 5d shows a detailof the atacamite–gypsum aggregate microstructure, show-ing the intergrowth of both minerals, and a layer of gypsumcan be seen in the middle of the elongated atacamite grain.

36Cl-to-Cl ratios

In order to gain insight into the origin of the Cl-rich fluidsfrom which atacamite precipitated, the 36Cl contents of ninesamples of atacamite from five ore deposits in the AtacamaDesert were determined together with two samples fromrecent rain events (Antofagasta and El Laco). Details of theMichilla (stratabound Cu or Manto-type), Antucoya-BueyMuerto (porphyry Cu), Radomiro Tomic (porphyry Cu),and Mantos de la Luna (manto-type) deposits can be found

in Trista-Aguilera et al. (2006), Maksaev et al. (2006,2007), and Cuadra and Rojas (2001), respectively. Thissuite of Cu deposits, all characterized by the developmentof atacamite-bearing oxide zones, covers a geographicrange between the Coastal Cordillera near the PacificOcean (Mantos Blancos, Mantos de la Luna, Michilla) tothe Central Depression (Antucoya-Buey Muerto, RadomiroTomic and Spence; Fig. 1). Results are reported as the ratioof 36Cl atoms to stable Cl atoms (36Cl to Cl; Table 3). Theatacamite samples show low 36Cl-to-Cl ratios, varying from11!10!15 to 28!10!15 at at!1. The highest and lowest 36Cl-to-Cl ratios are observed for Mantos Blancos (28!10!15)and Antucoya-Buey Muerto (11!10!15), respectively. Incontrast, the two rainwater samples have 36Cl-to-Cl ratiosof 867 and 2,247, respectively, a range to be expected forsamples from very arid climates and high altitudes (Bentleyet al. 1986).

Discussion

Solution salinities, gypsum supersaturation,and the formation of atacamite

A striking feature observed in all atacamite samplesanalyzed in this study is the close association of atacamitewith gypsum. As seen in Fig. 5a–e, atacamite–gypsumaggregates can be traced from a hand-sample scale down tothe nanometer range, and the textures observed at all scalessuggest that atacamite and gypsum are coeval. Contempo-raneous precipitation of gypsum and atacamite has also

Fig. 4 Histograms showing the distribution of salinities (in wt.% NaCleq) of fluid inclusions in atacamite from Mantos Blancos and Spencedeposits in northern Chile

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Table 2 Fluid inclusion salinity data for atacamite from Mantos Blancos and Spence deposits

Sample Tm ice (°C) Salinity,wt.% NaClequiv

Vapor, vol.% Sample Tm ice (°C) Salinity,wt.% NaClequiv

Vapor, vol.%

Mantos Blancos Spence

1 !7.1 10.6 10 1 !1.2 2.1 15!7.2 10.7 10 !1.4 2.4 15!7.0 10.5 20 !1.4 2.4 15!7.1 10.6 15 !1.3 2.2 20

2 !6.5 9.9 15 !1.3 2.2 10!6.5 9.9 15 2 !2.4 4.0 20!6.1 9.3 25 !2.5 4.2 25!6.3 9.6 20 !2.5 4.2 10!6.8 10.2 15 3 !1.9 3.2 10!6.4 9.7 10 !2.1 3.6 20!6.2 9.5 10 !2.0 3.4 20!6.0 9.2 10 !2.0 3.4 10!6.0 9.2 15 !2.6 4.3 15!6.2 9.5 15 !2.0 3.4 20!6.1 9.3 10 4 !1.7 2.9 25!6.6 10.0 10 !1.5 2.6 10!6.0 9.2 15 !1.5 2.6 20

3 !4.0 6.4 25 !1.5 2.6 20!4.9 7.7 10 !1.6 2.7 30!4.2 6.7 15 !1.5 2.6 10!4.4 7.0 15 !1.8 3.1 25!4.5 7.2 10 5 !1.3 2.2 15!4.0 6.4 20 !1.5 2.6 10!4.8 7.6 10 !1.2 2.1 10!4.2 6.7 10 !1.7 2.9 10!4.4 7.0 10 !1.5 2.6 30!4.6 7.3 15 6 !1.0 1.7 20!4.6 7.3 20 !1.0 1.7 20!4.5 7.2 10 !1.0 1.7 15!4.4 7.0 10 !1.0 1.7 25!4.5 7.2 10 !1.1 1.9 20!4.7 7.4 20 !1.1 1.9 25!4.5 7.2 15 !1.0 1.7 15!4.8 7.6 15 !1.0 1.7 10

4 !5.1 8.0 10 !1.2 2.1 20!5.8 9.0 15 !1.1 1.9 25!5.4 8.4 15 7 !1.7 2.9 30!5.5 8.6 15 !1.9 3.2 10!5.9 9.1 10 !1.8 3.1 20!5.7 8.8 10 !1.6 2.7 20!5.0 7.9 15 !1.8 3.1 20!5.2 8.1 15 !1.9 3.2 30!5.5 8.6 15 !1.9 3.2 10!5.3 8.3 10!5.0 7.9 10!5.2 8.1 10!5.6 8.7 10!5.3 8.3 15

Summary statistics n=48; <salinity>±σ=8.4±1.2 n=43; <salinity>±σ=2.7±0.7

Salinities were calculated using the formula reported by Bodnar (1993).

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Table 3 36Cl-to-Cl ratios in atacamite from ore deposits in the Atacama Desert in northern Chile

Material Sample Location Distance toPO (km)

Elevation(m a.s.l.)

36Cl–Cl(!10!15 atat!1)

AtacamiteMantos de la Luna ML-c 70° 13! W/22° 20! S 0.5 1,000 15.3±3.6Mantos de la Luna ML-c-a 70° 13! W/22° 20! S 0.5 1,000 16.6±4.1Mantos de la Luna ML-c-b 70° 13! W/22° 20! S 0.5 1,000 14.3±2.3Michilla SS-b 70° 07! W/22° 51! S 13 1,100 19.9±2.9Antucoya AB-b 70° 41! W/22° 37! S 35 1,700 11.1±1.7Mantos Blancos MB-b 70° 06! W/23° 22! S 40 1,100 26.5±5.7Mantos Blancos MB-b-a 70° 06! W/23° 22! S 40 1,100 28.0±4.4Spence SP-b 69° 18! W/22° 48! S 100 1,600 13.2±2.3Radomiro Tomic RT-a 60° 01! W/22° 28! S 150 2,950 16.1±1.9RainwaterAntofagasta; March 18, 2006 RWA-a 70° 22! W/23° 40! S 0.5 50 867±167El Laco; January 27, 2007 El Laco 67° 30! W/23° 42! S 210 4,850 2,247±315

The two rainwater samples were prepared using carrier.PO Pacific Ocean

Fig. 5 The atacamite–gypsumassociation from hand-samplescale down to the nanometerrange: a atacamite–gypsumveinlet from Spence deposit;b photomicrograph of fine-grained aggregates of atacamitein the veinlet shown ina (reflected light polarizing mi-croscopy); c bright-field TEMimages of atacamite aggregatesshowing a close relation withgypsum at the micron scale.Both minerals were identified byEDS analysis. Rectangle showslocation of detail in d; d detailshowing the mineralogical asso-ciation between atacamite andgypsum at the submicrometerrange, as seen under TEM ob-servation. Intercalations of gyp-sum (lighter) in atacamite(darker) can be observed; emagnification of the rectanglearea shown in d. At the nano-meter scale, atacamite and gyp-sum are found in closeassociation

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been reported by Glynn et al. (2006) in supergene alterationof submarine hydrothermal sulfides.

Present-day groundwater salinity (TDS) and sulfate(SO2!

4 ) trends (Fig. 2b) and fluid inclusion salinities inatacamite (Fig. 4) provide additional insights into thismineralogical association. At the Mantos Blancos andSpence deposits, the decrease in SO2!

4 of the groundwatersat the highest salinities (Fig. 2b) suggests that SO2!

4concentrations are controlled, in part, by the formation ofgypsum (Leybourne and Cameron 2006). Therefore, thesehighest TDS values reflect the salinities at which gypsum su-persaturated from groundwaters, ~100,000 and ~30,000 mg/Lfor Mantos Blancos and Spence, respectively. This argumentis supported by mineral saturation calculations reported byLeybourne and Cameron (2006, 2008). In the cited studies,determination of mineral saturation indices for groundwatersat Spence reveal that the highest salinity waters (west of thedeposit) are supersaturated with respect to gypsum and closeto (or exceeding) saturation with respect to secondaryminerals such as atacamite.

In order to make the salinities of the Na–Cl–SO4-richgroundwaters from Mantos Blancos and Spence (expressedas TDS in milligram per liter) comparable to the salinities offluid inclusions trapped in atacamite from both deposits(expressed as weight percent NaCleq), TDS values ofgroundwaters have been converted to weight percent NaCleqaccording to TDS*[weight percent NaCleq]=(Na[milligramper liter] + Cl[milligram per liter])10!4. When the gypsum-saturation salinities of groundwaters are compared with thefluid inclusion salinity data in atacamite for both deposits ina single diagram (Fig. 6), a striking correlation arises. Theaverage salinities of fluid inclusions in atacamite for MantosBlancos and Spence deposits (~7–9 and 2–3 wt.% NaCleq,respectively, Fig. 6, vertical lines) are strongly correlated tothe salinities at which gypsum precipitates from ground-waters in both deposits (corrected TDS* ~5–9 and 1–3 wt.%NaCleq, respectively, Fig. 6, trend line inflections). There-fore, as a first approximation, this correlation suggests thatthe range of salinities from which atacamite–gypsum formedis comparable to present-day, highly-saline gypsum- (andatacamite)-saturated groundwaters.

A deep, old, and saline source for atacamite formation

Although it has long been recognized that saline waters canform by evaporation of groundwaters of meteoric origin inhyperarid climates, recent studies report evidence ofalternative sources for the Cl-rich solutions required toform atacamite. Cameron et al. (2002, 2007) propose thatdeep saline formation waters (basinal brines), dewateredfrom sedimentary rocks underlying the Atacama Desert,can be forced to surface by tectonic pumping during

earthquakes. This hypothesis is supported by different linesof evidence, including: (a) the development of geochemicalanomalies in salt soils over orebodies that are stronglycorrelated with the distribution of fractures–faults below(Cameron et al. 2002; Palacios et al. 2005); (b) stableisotopic signatures that confirm involvement of seawater-like, deep formation waters–basinal brines that mix tovariable degrees with the less saline, regional groundwaterflow (e.g., δ37Cl in atacamite at Radomiro Tomic, Arcuriand Brimhall 2003; δD and δ18O in groundwaters atSpence, Leybourne and Cameron 2006).

The first measurements of 36Cl in atacamite, presentedhere, provide new lines of evidence concerning the originof the saline waters that formed atacamite (Table 3). Natural36Cl can be derived from cosmogenic, fissiogenic, andanthropogenic sources (Fehn et al. 1988; Snyder at al.2003; Fehn and Snyder 2005). The anthropogenic compo-nent is related to thermonuclear weapons tests, andconsidering the fact that atmospheric 36Cl has returned toprebomb ratios since about 1980 (Suter et al. 1987) we willfocus on the other two sources. The cosmogenic componentis formed by spallation by cosmic rays of Ar in theatmosphere and Ca and K in surface rocks and has been

Fig. 6 Sulfate content (SO4, milligram per liter) versus salinity(weight percent NaCleq, as calculated from TDS data, see text fordetail) of groundwaters from Mantos Blancos and Spence. The salinityat which groundwaters supersaturate with respect to gypsum (inflec-tion points) correlate well with the salinity range of fluid inclusions inatacamite from the same deposits (shown as shaded vertical intervalsdefined by the average (center line)±standard deviation of salinitiesfrom Table 2)

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used to determine groundwater ages and exposure ages ofsurface rocks (Purdy et al. 1996; Phillips et al. 2003). Intheory, the subsurface isolation time of water between 5!104 and 2!106 years can be determined using this method,considering that the half-life of 36Cl is 0.3 Ma (Fehn andSnyder 2005). On the other hand, the fissiogenic compo-nent is a result of capture by 35Cl (e.g., in pore waters) ofneutrons produced by the decay of U and Th isotopes (e.g.,in the host rocks). The buildup of 36Cl is a function of time,concentration of U and Th, and the presence of neutron-absorbing atoms competing with 35Cl. The presence offissiogenic 36Cl can be used to determine potential sourcesfor fluids (Rao et al. 1996) or the residence time of fluids ina given formation if the neutron flux can be estimated(Fehn et al. 1992; Fehn and Snyder 2005).

Despite the low 36Cl contents measured in atacamite inour samples (36Cl–Cl ~(11–28)!10!15, Table 3), its detec-tion suggests that 36Cl was inherited from the waters fromwhich atacamite formed. Thus, the data can be potentiallyused as an isotopic tracer for the waters involved in thegenesis of atacamite if the main source(s) component(s) for36Cl can be sorted out and assuming that the 36Cl budgetincorporated in atacamite (cosmogenic or fissiogenic inorigin) represents the 36Cl budget of the water from whichit precipitated (assuming, in this particular case, no fluid–mineral isotope fractionation and/or 36Cl loss or additionafter the formation of atacamite).

All atacamite samples show very low 36Cl-to-Cl ratios,close to the AMS detection limit of 1!10–15, comparable topreviously reported 36Cl/Cl ratios of deep formation watersand old groundwaters. Low 36Cl-to-Cl ratios, interpreted asin secular equilibrium with in situ (fissiogenic) 36Cl, havebeen reported in ultradeep mine waters at the WitwatersrandBasin in South Africa (Lippmann et al. 2003), deepgroundwaters from the Great Artesian Basin in Australia(Lehmann et al. 2003), waters from the German ContinentalDeep Drill Hole in Northern Bavaria (Fehn and Snyder2005), deep formation waters from the Fruitland Formationin Colorado and New Mexico (Snyder et al. 2003; Snyderand Fabryka-Martin 2007), deep groundwater from theNorthern Sahara (Guendouz and Michelot 2006), and theEast Irish Sea Basin (Metcalfe et al. 2007). In all these cases,low 36Cl-to-Cl ratios (<30!10!15) reflect subsurface, fissio-genic production of 36Cl in secular equilibrium indicating anage in excess of 1.5 Ma, or five times the half-life of 36Cl.Although the meteoric component cannot be discarded,disentangling the cosmogenic (meteoric) 36Cl signal istypically difficult, in particular for deeper and more salinegroundwater, mainly due to subsurface production of 36Cl byneutron capture of 35Cl and transport from adjacent aquifersor pore water reservoirs (Lehmann et al. 2003).

A potential check on the dominant source of chlorine inthese deposits is possible by relating the results to the

distance from the ocean and altitude. 36Cl-to-Cl ratios inrainwater and resulting surface or groundwaters show astrong dependence on the distance from the ocean becausechlorine in the atmosphere is derived predominantly fromsea spray. The long residence time of chlorine in the oceansresults in marine 36Cl-to-Cl ratios below 0.5!10!15 (i.e.,below the detection limit of AMS). In the atmosphere, 36Cl-to-Cl ratios increase with altitude because the production rateof 36Cl diminishes as cosmic rays are attenuated passingthrough the atmosphere. As a result of these processes, 36Cl-to-Cl ratios in surface waters generally increase strongly withdistance from the oceans and with altitude (e.g., Bentley etal. 1986), also observed for the two rainwater samplesmeasured for this study. In contrast to these surface watersamples, results for the atacamite samples do not show acorrelation with distance from the oceans or altitude,suggesting that 36Cl in these deposits is mainly derived notfrom cosmogenic but from fissiogenic sources.

In order to further evaluate the role of fissiogenic 36Cl asa potential source for 36Cl in atacamite, we related theabundances of U and Th in the host rocks of each oredeposit (reported by Dietrich 1999; Oliveros 2005; Ramírezet al. 2008) to their respective 36Cl-to-Cl ratios in atacamite(Fig. 7). There is a positive correlation between U + Thconcentration in the host rocks and 36Cl-to-Cl ratios inatacamite, with the highest 36Cl–Cl values at MantosBlancos and the lowest at Antucoya and Spence. Thistrend supports our interpretation of the low 36Cl-to-Cl ratiosin atacamite (11–28!10–15 at at!1) as representing subsur-face production of fissiogenic 36Cl in secular equilibriumwith the solutions involved in the atacamite origin(Lehmann et al. 2003; Lippmann et al. 2003). Becauseatacamite does not contain U or Th itself, the production of36Cl is no longer supported once chlorine has entered theminerals and the 36Cl-to-Cl ratio starts to decrease with age.The fact that we still found measurable 36Cl in atacamiteindicates then that the formation of these minerals wasrelatively recent. The 36Cl data strongly suggest that thechlorine in the saline waters related to atacamite formationis old in origin (>1.5 Ma) but that the atacamite formationoccurred less than 1.5 Ma ago (or 5! half-life of 36Cl).

Although more than one mechanism may be involved information of atacamite in the hyperarid Atacama Desert, theresults support the tectonic pumping model proposed byCameron et al. (2002, 2007). In the context of this model,saline formation water from the dewatering of the sedi-mentary basin is carried up through the deposit, modifyingthe existing oxide assemblage, creating an atacamite-stableassemblage. During earthquakes, the saline water is forcedto the surface of the covering gravels, creating salineanomalies plus Cu derived from the deposit. Palacios et al.(2005) reported Cu anomalies in soils above the MantosBlancos copper deposit. Although the Cu anomalies in

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surface salt samples (gypsum + anhydrite + halite) over theore bodies are spiky, their contrast with the background Cuvalues is remarkable. The same study reports a goodcorrelation between Cu and Na where the concentration ofCu is high and shows that Cu anomalies spatially correlatewith faults that crosscut Cu mineralization. The previousobservation is confirmed by the presence of salt efflor-escences (halite + gypsum) that contain atacamite andchalcanthite at the surface along major active faults near thedeposits (e.g., Salar del Carmen fault close to the MantosBlancos), strongly suggesting that Cu was brought to thesurface by saline waters. Copper anomalies from Miocenealluvial gravel soils above the Spence porphyry copperhave also been reported by Cameron and Leybourne(2005). The anomalous Cu values developed over the oredeposit show the same spiky response documented byPalacios et al. (2005), and the peaks are observed in zoneswhere the partly salt-cemented gravels are fractured.Enzyme leach and deionized water analyses, which mea-sure water-soluble constituents, evidence Cl and Naanomalies in gravel soils above the Spence deposit andalso over fractured zones (Cameron et al. 2002; Cameronand Leybourne 2005). These fracture zones over the depositare interpreted to represent seismically active basementfaults that have propagated through the gravel coverCameron and Leybourne (2005), suggesting an importantrelationship between the ascent of saline, deep, and oldwaters and atacamite formation in hyperarid regions such asthe Atacama Desert.

Summary and conclusions

The formation of atacamite in oxide zones in Cu depositsfrom the Atacama Desert in northern Chile is related togypsum-saturated saline groundwaters (TDS*>20,000 mg/Lor 2 wt.% NaCleq), as confirmed by fluid inclusions in

atacamite and present-day groundwater chemistry. Thesalinity at which groundwaters at the Spence and MantosBlancos saturate with respect to gypsum (1–3 and 5–9 wt.%NaCleq, respectively) is in the range of salinities measuredin fluid inclusions in atacamite, suggesting a closeconnection between gypsum saturation and atacamiteformation. This genetic link is confirmed by observationsat different scales that confirm a close mineralogicalassociation between these two minerals, from sample scaledown to the nanometer range. In addition, 36Cl data inatacamite show that the low but detectable 36Cl/Cl ratiosmeasured in atacamite (11–28!10–15 atat!1) are probablyrelated to fissiogenic production in the subsurface, suggestingthat atacamite in these deposits is produced from chloride insaline waters with ages in excess of 1.5 Ma but that theformation process itself occurred relatively recently (<1.5 Ma).

These results, coupled with previous studies of ground-water chemistry and stable isotope distributions, suggest adeep origin for the saline waters responsible for theformation of atacamite. Our results provide new constraintson the age of atacamite in ore deposits from the Atacama

Fig. 8 Atacamite in miner boots recovered from a deep gallery (150 mdeep) in early 1900s workings around the late Paleocene Sierra GordaCu-porphyry deposit (photo courtesy of the Geology Museum of theDepartment of Geosciences, Universidad Católica del Norte, UCN,Antofagasta, Chile). Atacamite (green) was confirmed by powder X-raydiffraction analysis at UCN

Fig. 7 Variation of 36Cl-to-Clratios in atacamite as a functionof the U + Th concentration ofhost rocks, for various depositsin the Atacama Desert in north-ern Chile. AB = Antucoya-BueyMuerto, SP = Spence, ML =Mantos de la Luna, RT =Radomiro Tomic, SS = Michilla,and MB = Mantos Blancos

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Desert and support the recent notion that supergeneenrichment may have occurred more recently (<1.5 Ma)than many models for the climate of northern Chile suggest(e.g., 9–14 Ma, Alpers and Brimhall 1988; 3–6 Ma, Hartleyand Rice 2005). The results suggest that formation ofatacamite in hyperarid climates such as the Atacama Desertis an ongoing process (e.g., Fig. 8) that has occurredintermittently, since the onset of hyperaridity.

Acknowledgements Support for this study was received fromComisión Nacional de Ciencia y Tecnología de Chile grant #1070736 to Carlos Palacios, Martin Reich, and Miguel Angel Parada.We are grateful to BHP-Billiton for logistical assistance and forproviding access to the Spence deposit and specifically to Martin J.Williams, Mario Sáez, Walter Ruf, and Jorge Peña for their helpduring open-pit and core sampling. We also thank Anglo American forproviding us with samples from Mantos Blancos and for releasinganalytical data for publication (research contract D-1012). Thetransmission electron microscope used in this work was acquiredunder the Mecesup grant UCH-0205. We thank Christian Nievas andMarcela Robles for TEM sample preparation. Z. Lu at the Universityof Rochester prepared the AgCl targets; M. Caffee and the AMSgroup at PrimeLab, Purdue University, carried out the 36Cl determi-nations; we appreciate their efforts. We acknowledge Bernd Lehmannfor his helpful and constructive review of the manuscript.

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